Unify signature creation

[twiss]

Make signature packet creation more uniform:

• Consistently add an Issuer Fingerprint to all signatures.
• Consistently add a Signature Expiration Time when config.SigLifetimeSecs is set.
• When generating a signing subkey, use the version of the subkey to determine the version of the cross-signature, rather than the version of the primary key.
• Similarly, fix the Issuer Key ID of signing subkey cross-signatures to refer to the subkey instead of the primary key.